Magnetron pilot cavity resonator



Qct. 31, 1950 w. v. SMITH 2,527,770

MAGNETRON PILOT CAVITY RESONATOR Filed Oct. 10, 1945 FIG] 23 Y 25 l I 2 "I"? i INVENTOR.

WILLIAM V. SMITH BY wax-M94414.

ATTORNEY Patented Oct. 31, 1950 UNITED STATES T O 1 CE MAGNETRON PILOT CAVITY RESONATOR Application October 10, 1945,Serial'No. 621,563

(CL 315F359.)

6 Claims. 1

The invention relates to improvements in electrical apparatus and more particularly to improvements in radio frequency, oscillators such as magnetrons.

In one method of frequency modulating a magnetron a thermally tuned electron gun within one of the magnetron cavities has been employed. This method has a number of disadvantages; tuning is sluggish and only very low rates of frequency modulation are possible by this method.

An objectof the presentinvention is to provide a magnetron oscillator which is mechanically tunable over a substantial range of center frequencies. Another object is to provide for electronic frequency modulation at any given center.

frequency, and for maintaining the degree of frequency modulation substantially constant over the range of centerfrequencies to which the magnetron may be tuned.

Other objects and advantages of the invention will be apparent during the course of the following description.

These objects are accomplished by coupling an external auxiliary cavity resonator, here called a magnetron pilot cavity resonator, to one of the cavity resonators of the power generating magnetron. The resonant frequency of the pilot cavity resonator is varied by mechanical adjustment and by a cloud or beam of electrons exposed to the high-frequency electric field within the cavity. A change in the'resonant'freouency of the pilot cavity resonator couples positive or negative reactance into the power generating magnetron system, changing its resonant frequency and therewith the frequency of the generated power. A magnetic-field parallel to the electron beam is superposed'to concentrate and stabilize the electron flow.

In the accompanyingdrawing forming a part of this specification and in which like numerals areemployed to designate like parts throughout the same;

' Fig. 1 is a schematic viewo'f a magnetron embodying the invention.

Fig. 2 is a cross sectional view taken on the line 2-2 of Fig. l. a

In the drawing Fig. 1, wherein for the purpose of illustration is shown a preferred embodiment of the invention, the numeral ill designates a magnetron of the plural; cavity type including a cylindrical cathode surrounded by radial anode segments l2. The anode segments form the lateral walls for the individual cavities l3. Alternate anode segmentsxare connected by straps I4 of which only one is shown.

The magnetron output .may be coupled toa '2 load in any conventional manner. For example, a loop may be provided'in one of the magnetron cavities to accomplish this function.

The magnetron ill is coupled to a magnetron pilot cavity resonator I5 by means of an iris coupling l6. The-magnetronpilot cavity resonator l5 is-a half wavelength section of H- shaped wave guide,

Referring toFig. 2 which is across sectional viewta'ken through themagnetron pilot cavity resonator on the line 22 (Fig. '1'), the numerals 2i} and 2| represent opposing'surfaces which cooperate to define the capacitivegap '22. The dimension h. of capacitive gap 22 maybe varied by regulating the position of capacitive plunger 23 by means of a hand knob 28 which is rigidly fastened to the plunger spoke 24 and turns on a threaded guide 25. Pole pieces '26'and 21 create a magnetic field substantially parallel to and between surfaces '20 and 2|.

An electron gun including a filament30, cathode 3|, control grid'32, screening grid 33 and collecting anodeM sends a beam of electrons 35 [through the capacitive gap 22, said beam of electrons 35 being substantially parallelto and between surfaces 20 and 2|.

The operation of the frequency modulated magnetron which is a preferred embodiment of the invention is as follows:

The magnetron |0 oscillates upon the application of the operating voltage between cathode II and anode segments l2. The frequency of the magnetron oscillationsmay be varied by varying the resonant frequency of the magnetronpilot cavity resonator l5. The stabilization factor S is a' measure of the influence of thepilot cavity resonator l5 on the magnetron III, as. indicated in the following equation:

.AF -S-1 1) Iris where-AF is the'frequency change of the combined system of magnetron l9 and cavity resonator l5, and M is the corresponding frequency change.

for the cavity resonator l5 alone.

The resonant frequency of the cavity |5 may be varied by mechanical tuning means (6. g., ca-

pacitive plunger 23) and by electronic means f density of electron beam 35 may be modulated by a voltage applied to control grid 32. Customary plate-cathode, screen-cathode and gridcathode direct current voltages are applied to the elements of the electron gun.

As shown in Fig. 2, the screening grid 33 and collecting anode 34 are both electrically connected to the walls of the cavity resonator l5. Inasmuch as these walls are customarily grounded, the cathode 3| will be operated at some negative voltage (with respect to ground) to maintain the required potential differences between cathode 3I, screening grid 33 and collecting anode 34. Although the screening grid 33 and collecting anode 34 are here shown as being at the same potential, the screen grid 33 may be operated at a somewhat higher or lower potential than that of the collecting anode 34 if desired.

The modulation of the density of electron beam 35 by the voltage applied to control grid 32 will-electrically modulate the frequency of said cavity [5 by variation of the electron loading in the capacitive gap 22. Furthermore, by proper design of relations between the stabilization factor S and the magnetic field intensity, as will appear more in detail hereafter, it is possibleto maintain the degree of frequency modulation substantially constant over the range of mechanical frequency adjustment of the cavity resonator l5; said mechanical frequenc adiustment being made by movement of the capacitive tuning plunger 23. I

The electron loading effected by the electron beam 35 within the capacitive gap 22 is augmented as'a result of the complex helical motion which the electrons describe under the influence of the combined magnetic field (generated by pole pieces 26 and 2])"and the electric field between surfaces 29 and 2! (the electric field being thealternatingelectric field of frequency'corresponding to the frequency of the magnetron H1 and cavity 5). The magnetic field created by pole pieces 26' and-2l is parallel to the electron beam 35. The alternating electric field generated between surfaces 20 and 2| is at right angles tothe magnetic field. Under the action of the combined electric and magnetic field; the electrons describe a complex motion, which is that of. ahel-ix of constantly increasing diameter to some maximum diameter, thence of constantly decreasing'diameterto a zero diameter point or node, thence to the maximum diameter again maintaining this sequence of motion whilepassing between the surfaces 20 and 2|. Alternatively, the motion may be termed an increasing and decreasing "Archimedean spiral helix} in analogy to the customary circular helix. Two

frequencies are relevant in the discussion of this motion. They are F, the frequency of the alternating electric field between surfaces 20 and 2|, and F, the cyclotron frequency, (frequency of rotation of an electron of charge e and mass m in a magnetic field of intensity B).

The frequency of the electrons within the "Archime'dean spiral helix.is

the. frequency of the zero diameter nodes is F-''F. t t

'4 The effect of this complex motion of the electrons is to increase the effective alternating current electron density within the capacitive gap 22. The variation of the electron density within the gap 22 by means of the control grid 32 results in a frequency modulation of the cavit l5 and of the magnetron Hi.

fhat the design shown in Figures 1 and 2 will permit the frequency modulation of the cavity I 5 to remain substantially constant over the tuning range of the cavity 15 caused by the adjustment of the mechanicall adjusted tuning plunger 23 will now be shown.

A table of the various symbols used in the discussion follows:

k1, In, etc, are constants.

i=5 where I is the current and h the gap spacing in gap 22. (The symbol E means is equivalent to) where B is the magnetic field strength generated by poles 25 and 27 and H is the cyclotron field strength.

e/m is rati of charge to mass for the electron.

F is the center frequency of the magnetron-cavity combination.

I is the resonant center frequency of cavity resonator [5 alone.

F and f are adjusted to be equal at the center of the mechanical tuning range.

To a first approximation the equation governing the electronically produced frequency change AFe (caused by electronic loading alone) is given below: I

The adjustment of the height h of gap 22 by the mechanical tuning control 23 results in a mechanicall produced frequency change of the resonant frequency of cavity l5 as indicated in the following equation.

The variation of the mechanical tuning control 23 in addition to varying the center frequency F about which the electronic frequency modulation takes place will also vary {3 since and would result in a change of AFe, the degree of frequency modulation (Equation 3). However, the variation of h likewise causes a. counterbalancing change in since elm '5 Taking differentials inEquation: 7=

. 1 5115-1). 1; (8) M r A'h'f" or V agar (9) j 'B1 From Equation 6 'B 5- 1 (11) 1 1 B'- 8 From Equation '5 9E" ('12) B F E, All Al. (13) F ix f) F From Equation 4 Substituting in Equation 13, the equivalents determined fromcEquations 1 and 14 andremembering, that By substituting the equivalent of B from Equation 5, and solving for B.

Equations 18 and 19 hold exactly only at one value of h, since ,8 varies with h. However, the Variation of the ratio with h is minimized by satisfying Equation 18 at the center value of h.

In the customary design problem for a frequency modulated magnetron according to the present invention, S is known (usually in the range of 1.25-2.00), and the magnetic field intensity B is designed to satisfy equation 19 at the center frequency of the tuning range. (Adjustment of B rather than F in Equation 19 is chosen since F is predetermined.) It is desirable to make B variable over a slight range (in the completed device) to more accurately satisfy the requirement of Equation 19.

In the alternative event that 6 must have a certain value (to secure a required tuning range), S may be designed to satisfy Equation 18.

6 v For the special case of the cavity resonator alone, (i. e. disconnected from the magnetron) Equation 19 reduces to:

, 41rF (20) e/ m By satisfying Equation '-2-0'at the midfreque'ncy of the cavity resonator, a substantially constant degree of electronic frequency modulation of" the cavity resonator frequencyover the range of the mechanical tuning is possible (in complete analogy to 'the'c ase ofthe magnetron-resonator combina'tion).

It is'obviou's that the cavityresonator embodying the principle of the present invention is not limited solely to combination with magnetron 05-: cillators, but may be used to control the'frequen'cy of various other radio frequency oscillators, the frequency of which correspond to the frequency of said cavity resonator.

The advantageous features of'thls invention are summarized as follows:

a. Mechanical tuning of center frequency.

b. Electronic frequency modulation of frequency.

c. Substantially constant degree of electronic frequency modulation over the mechanical tuning range.

While there has been described What isatpres ent considered to be'thepre'fered embodiment of thisinvention, it will be obvious'to those skilled in the art thatvarious-'changes-and modifications may be mode therein without departing from the scope of the invention as set forth in the appended claims.

What is claimed is:

1. A magnetron pilot cavity resonator coupled to a magnetron, said resonator having an adjustable capacitive gap formed by surfaces therein, electron means for varying the resonant frequency of said resonator, said means including a cathode, an anode and a control grid adjacent said gap and adapted to pass an electron beam therethrough substantially parallel to said surfaces and to vary the flow of electrons in said beam, and means for generating a magnetic field substantially parallel to and including said electron beam whereby the density of said electron beam may be modulated thereby modulating the resonant frequency of said resonator and introducing a corresponding frequency modulation in the output of said magnetron.

2. A magnetron pilot cavity resonator coupled to a magnetron, said resonator comprising, a wave guide closed at one end and open at the other to one of the magnetron cavity resonators, said wave guide having surfaces therein definin a capacitive gap, adjustable mechanical tuning means for varying the spacing of said surfaces whereby the center frequency of said resonator may be varied, electron means adapted to pass an electron beam through said gap substantially parallel to said surfaces, said electron means including a cathode, an anode and a control grid adjacent said gap, and means for generatin a magnetic field substantially parallel to and including said electron beam whereby voltages applied to said control grid will vary the electron density of said electron beam thereby varying the resonant frequency of said resonator and the frequency of said magnetron.

3. A magnetron pilot cavity resonator havin a capacitive gap formed by surfaces therein, means for adjusting the acing between said surfaces,

means positioned to pass an electron beam of variable intensity through said gap substantially parallel to said surfaces to vary the resonant frequency of said resonator, and means for generating a magnetic field substantially parallel to and including said electron beam to stabilize and concentrate said electron beam.

4. Atmag-netron pilot cavity resonator having a capacitive gap formed by surfaces therein, mechanical tunin means for adjusting the spacing of said surfaces for varying the center frequency of said resonator, electronic means for varying the resonant frequency of said resonator, said means including a cathode, an anode, and a grid adjacent said capacitive gap and adapted to pass an electron beam therethrough substantially parallel to said surfaces and to vary the intensity of said beam, and means for generating a magnetic field substantially parallel to and including said electron beam.

5. A magnetron pilot cavity resonator, said resonator having a capacitive gap formed by surfaces therein across which high frequency electric fields are impressed, electron means for varying the effective capacity of said gap, said means including a cathode, an anode, and a grid adjacent said capacitive gap and adapted to pass an electron beam therethrough substantially parallel to said surfaces whereby the control of the density of said electron beam may be utilized to vary the resonant frequency of said resonator, and means for generating a magnetic field substantially parallel to and including said electron beam.

6. A magnetron pilot cavity resonator, said 8 resonator havin a capacitive gap formed by surfaces therein across which a high frequency electric field is impressed, electron means adjacent said capacitive gapfor varying the efiective capacity of said gap, said electron means including a cathode, an anode, and a control grid, said electron means adapted to pass a beam of electrons through said gap at right angles to said electric field whereby the electron density of said electron beam within said gap maybe varied, and a pair of magnet poles for producing a magnetic field parallel to the axis of and including said electron beam whereby said electron beam is stabilized and concentrated in said gap.

WILLIAM V. SMITH.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED sTATEs PATENTS 

